1. Field of the Invention
This invention relates to a lens system for a miniature image pickup device, and more particularly to a retrofocus lens system to be mounted on a miniature image pickup device such as a video camera using an image pickup element.
2. Description of the Prior Art
Recently, as a video camera or a still-video camera (a video camera which is also capable of taking a still photo) using a solid state image pickup element comes into wide use, demand for a retrofocus lens system which is small in size and large in aperture ratio is increasing. Especially, there is an urgent necessity of a clear retrofocus lens system having a large aperture ratio in order to meet demands for miniaturization of such image pickup devices.
Further in such a miniature image pickup device, since a lowpass filter for cutting high frequency components of light such as those of quartz plate is disposed between the lens system and the image plane, the retrofocus lens should have a long back focus.
Thus there is a demand for a large aperture retrofocus lens system which is small in size, large in aperture ratio and long in back focus.
In order to meet such a demand, there have been developed various lens systems as disclosed in Japanese Unexamined Patent Publication Nos. 63(1988)-81414, 2(1990)-85816, 60(1985)-26919, 59(1984)-232310 and the like.
However increasing the aperture ratio of a lens system (decreasing the f-number) and increasing the back focus generally conflict with each other and it has been difficult to meet both the requirements on the aperture ratio and the back focus while keeping small lens aberrations.
For example, in the lens system disclosed in Japanese Unexamined Patent Publication No. 63(1988)-81414, though the back focus can be increased up to about 1.2f, the f-number cannot be reduced below 1.65 since the spherical aberration becomes too large if the f-number is reduced up to about 1.2. In the lens system disclosed in Japanese Unexamined Patent Publication No. 2(1990)-85816, though the f-number is about 1.44 and relatively small, the angle of view is too small and the back focus is short. Further when the f-number is reduced up to about 1.2, the coma and the astigmatism deteriorate. Further in Japanese Unexamined Patent Publication No. 60(1985)-26919, there is described a lens system whose the f-number is 1.2. However in this lens system, correction of the spherical aberration, the coma and the astigmatism is not sufficient.
In the lens system disclosed in Japanese Unexamined Patent Publication No. 59(1984)-232310, the f-number can be as small as 1.2 and the back focus can be long without deteriorating the aberrations, whereby a clear lens system which permits the distance between the final lens surface and the image plane to be long.
However since, being of seven lenses, the lens system is not satisfactory from the viewpoint of reducing the lens size and lowering the manufacturing cost, and accordingly, development of a six-lens lens system having an equivalent performance has been required.